The invention relates to an optical relay comprising a target plate of an electrically insulating material and allowing light to pass in a manner dependent on an electric field parallel to the direction of propagation of said light, means for scanning a first surface of said target plate by means of an electronic beam controlled by a wehnelt electrode, an anode suitable for collecting secondary electrons emitted by the action of said beam, an optically transparent and electrically conducting thin plate engaging a second surface of the target plate, said thin plate receiving the visual information electric signal from an amplifier which supplies a video signal realising a potential modulation of the target plate which is constituted by a material becoming ferroelectric below a given temperature referred to as the Curie temperature in the proximity of which the optical relay is operative.
An optical relay of this type for projection television is described in French Pat. Nos. 1,473,212 (U.S. Pat. No. 3,520,589) and 1,479,284. For a better understanding of the invention the operating principle of this optical relay will be described hereinafter. More extensive information can be obtained from the above-cited documents.
Within the scope of the invention an electric signal having a temporal variation and representing visual information is transformed into a visible image. It is known that this is one of the roles of a television receiver.
In the image tube of such a receiver the electron beam fulfills the three fundamental functions of this transformation:
f1-it supplies the energy which should appear in a luminous form: the luminous power of the tube is thus always less than the power transferred by the beam; PA1 f2-it realises the spatial scanning of the image surface; PA1 f3-it supplies the visual information. PA1 piezo-electric coefficients, PA1 electro-optical coefficients, PA1 dielectric constants .epsilon..sub.x and .epsilon..sub.z.
Because of the functions f2 and f3, inter alia, the power of the beam and thus the brightness of the image cannot be improved as much as would be necessary for projection on, for example a large screen.
This is why it has been proposed to separate the function f1 of, for example an arc lamp, from the functions f2 and f3 of what is referred to as an "optical relay". In this proposal a crystal is used having an electro-optical effect referred to as the "Pockel's effect". A double-acid potassium phosphate crystal KH.sub.2 PO.sub.4 hereinafter referred to as KDP is suitable for this purpose.
This effect may be broadly described as follows: when the electrically insulating crystal is exposed to an electric field parallel to its crystal axis c (the three crystal axes a, b and c are mutually orthogonal, with the axis c being the optic axis in this case), the refractive index n of this crystal for light rays propagating in the c direction with linear polarization in the ab plane depends on the direction of polarisation. If X and Y designate the bisectrices of the axes a and b and if the parameters of the crystal with respect to these different directions are designated by the letters used for said directions, it can be said that the diagram of the refractive indices in the ab plane forms an ellipse having axes X and Y instead of forming a circle, and that the difference between said indices n.sub.x -n.sub.y is proportional to the electric field applied. It follows therefrom that if the incident light rays are polarized parallel to the axis a, for example, the intensity I of the light passing through an output polariser is I=I.sub.o sin.sup.2 kV if the direction of polarization of this polarizer is parallel to the axis b, and I=I.sub.o cos.sup.2 kV if this direction is parallel to the axis a, I.sub.o is the intensity of the incident light when no parasitic absorption occurs, V is the electric potential difference between the two planes of the crystal, and k is a coefficient depending upon the crystal material used.
In order to obtain a projected picture by means of a lamp via this device, it is sufficient, as stated above, to apply an electric field parallel to the axis c and to cause the value of this field at any point of the target plate to correspond to the brightness at the corresponding point of the picture to be obtained. For this purpose an electron beam emitted by an electron gun and traversing conventional deflection members scans the target plate and thus performs the function f2. The function f3, that is to say, the control of the electric field, is also performed by the beam in the following manner.
When the electrons of the beam arrive at the surface of the target plate, they cause secondary electrons if their energy lies within the desired limits and if the anode potential is sufficiently high, the number of secondary electrons exceeding that of the incident electrons. As a result the electric potential of the point of incidence is raised so that the potential difference between the anode and this point is reduced. If the electrons of the beam strike this point in a sufficient number, this potential difference becomes negative and reaches such a value (for example -3V) that each incident electron releases only a single secondary electron. The potential of this point thus reaches a limit value with respect to the anode potential. In accordance with the scanning rate the intensity of the beam is to be sufficiently high. If the anode potential is constant, each passage of the electron beam fixes, as stated above, the potential of any point A of the surface at a value V.sub.o independently of this point and at the instant of passage. However, the corresponding electric charge at this point depends on the potential of the control electrode arranged in the proximity of the other side of the target plate.
If VA is the potential of this electrode at the instant of passage, this charge is proportional to V.sub.o -VA, in which VA represents the value of the visual information signal at the instant of its passage.
The target plate whose birefringence depends on the electric field is formed by a single crystal of KDP in which about 95% of the hydrogen is in the form of heavy hydrogen (deuterium).
With a given thickness of the crystal the Pockel's effect is proportional to the charges produced on the crystal sides and hence, with a given control voltage, to the dielectric constant of the crystal. This is why a target plate is used which is formed by a crystal becoming ferro-electric below a given temperature referred to as the Curie temperature and why operation is advantageous in the proximity of this temperature, because the dielectric constant then attains very high values and the optical relay can function by means of readily obtainable control voltages (the Pockel's effect is proportional to the product .epsilon.V).
The most frequently used crystals exhibiting this phenomenon are acid salts of the KDP type in the quadratic crystal class the optic axis of which is parallel to the crystal axis c. Its Curie temperature is about -53.degree. C. Above this Curie temperature the DKDP is a quadratic crystal in the symmetry class 42 m and it has a para-electric behavior. Below the Curie temperature the DKDP becomes orthorhombic, symmetry class mm2, and exhibits a ferroelectric behavior: locally there is spontaneous polarization and appearance of ferro-electric domains.
At the ambient temperature the crystal is anisotropic, but in the proximity of the Curie temperature the anisotropy becomes extremely important. The change of state is accompanied by abrupt variations in the physical properties along the axes of the crystal:
Thus the dielectric constant .epsilon..sub.z goes from a value of approximately .alpha.at the ambient temperature to a value of 30,000 at the Curie temperature.
It is known that from an electrical point of view the apparent thickness e of the DKDP crystal is EQU e=E.(.epsilon..sub.x /.epsilon.'.sub.z).sup.0.5
The target plate appears to be thinner as the ratio .epsilon..sub.x /.epsilon.'.sub.z is smaller, where .epsilon.'.sub.z has the value of .epsilon..sub.z when the crystal is blocked mechanically. In an optical relay the monocrystalline DKDP plate having a thickness E of approximately 250 microns is firmly adhered to a rigid support: a fluorine plate having a thickness of 5 mm.
The optical relay target plate is thus generally cooled to -51.degree. C., that is to say to a temperature which is slightly higher than the Curie temperature. In these conditions .epsilon..sub.x /.epsilon.'.sub.z =1/9 and the apparent thickness of the crystal is approximately 80 microns, which gives the optical relay a satisfactory picture resolution. Below the Curie temperature this ratio .epsilon..sub.x /.epsilon.'.sub.z is still smaller, which results in a much better picture resolution.
When such an optical relay is used for projecting information such as marks and/or alphanumeric characters, their visibility is greatly dependent on the scanning format. This becomes manifest when the scanning standard is changed from 625 lines to 1025 lines. The increase in the number of horizontal scanning lines is then accompanied by a considerable attenuation of the contrast in the horizontal marks, which impedes the readability of the projected alphanumeric characters.
This attenuation in the contrast of the horizontal marks as a function of narrowing the scanning lines also becomes manifest in the case of white marks on a black background or black marks on a white background.
If the horizontal scanning is compressed, it is the vertical marks which become less visible, whereas the vertical marks become much larger and much more luminous if this horizontal scanning is dilated.
The same inconveniences likewise appear, though to a lesser degree, in the case of television pictures.